Bone metastasis of cancer is conventionally diagnosed by examining clinical symptoms of the patient or images taken by simple radiography, bone scintigraphy, CT, MRI, etc. From a visual viewpoint, bone metastases are classified as a dissolution type, a hardening type or a mixed type depending on the balance between bone dissolution and formation at the site of bone metastasis. While image diagnoses are highly reliable and useful, they are generally too expensive to be used for screening and monitoring purposes.
With the recent advances in the study of bone metabolism, various markers have been developed as indices of bone metabolism. Markers of bone formation and resorption are separately listed in Table 1. Attempts are being made to diagnose bone metastases of cancers using those bone metabolic markers (Koizumi, M. et al., Bone Metabolic Markers in Bone Metastases, J. Cancer Res. and Clin. Oncol., 121:541-548, 1995).
TABLE 1Markers of bone formation(1)Type I procollagen peptides proliferationC-terminal propeptide (PICP)N-terminal propeptide (PINP)(2)Alkali phosphatases matrix formationtotal alkali phosphatase (ALP)bone alkali phosphatase (BALP)(3)Osteocalcin (OC) mineralizationC-terminal fragmentsintermediate portionsintactMarkers of bone resorption(1)pyridinium cross-linkstotal urinary pyridinoline•deoxypyridinoline (HPLC method)free urinary deoxypyridinoline (fDPD)(2)pyridinium crosslinked collagen peptide fragmentsserum C-terminal telopeptide (ICTP)urinary C-terminal telopeptide (CTx)urinary N-terminal telopeptide (NTx)(3)Tartrate-resistant acid phosphatase (TRAP)(4)Galactosyl hydroxylysine (GHYL)(5)Hydroxyproline(6)N-terminal osteocalcin
Most of the bone metabolic markers have as their rationale the measurement of metabolic products that are released into blood and urine in the process of formation and absorption of type I collagen which accounts for 90% of the bone matrix. To be more specific, type I procollagen which is synthesized during bone formation releases C- and N-terminal propeptides when it is converted to type I collagen and these propeptides serve as markers of bone formation. In the process of bone resorption, the type I collagen in the bone matrix undergoes metabolism to be released into blood and urine; the measured blood and urine levels of the released type I collagen serve as markers of bone resorption.
Bone formation is known to consist of three major phases depending upon the stage of proliferation and differentiation of osteoblasts; the first phase is where oesteoblasts proliferate and the matrix forms, the second phase is for matrix maturation and the third phase is for calcification, and different markers are known to appear at different phases (Stein, G. S. et al.: Relationship of Cell Growth to the Regulation of Tissue-Specific Gene Expression during Osteoblast Differentiation, FASEB J., 4:3111-3123, 1990).
In the phase of osteoblast proliferation and matrix formation, type I collagen forms actively and C- and N-terminal propeptides appear in the blood. In the phase of matrix maturation, bone alkali phosphatase (BALP) is generated actively, causing BALP to be secreted into the blood. At the stage of calcification, osteocalcin (OC) appears. Bone formation is accelerated in OC-deficient mice, suggesting that OC works as a suppressant of bone formation (Ducy, P. et al.: Increased Bone Formation in Osteocalcin-Deficient Mice; Nature, 382:448-452, 1996).
In the box of “Markers of bone formation” in Table 1, “(1) proliferation” corresponds to the phase of osteoblast proliferation and matrix formation, “(2) matrix formation” to the phase of matrix maturation, and “(3) mineralization” to the phase of calcification.
While there are various markers of bone formation, they frequently behave differently depending upon the state of the disease and it is important to realize specifically in which phase each marker appears.
There are also various markers of bone resorption and as with the markers of bone formation, metabolic products of type I collagen are currently drawing special attention. In type I collagen, collagen of a triple-stranded structure occurs crosslinked with pyridinoline and deoxypyridinoline, so when it is destroyed upon bone resorption, pyridinoline and deoxypyridinoline cross-links having various sizes of N- and C-terminal amino acids attached thereto are released into the blood.
The measurements of resorptive markers include that of cross-links alone (urinary pyridinoline and deoxypyridinoline that are measured as free entities), that of cross-links including C-terminal amino acids (CTx and ICTP), and that of cross-links including N-terminal amino acids (NTx). For generalized details about bone metabolic markers, see the review article by Calvo et al. (Calvo, M. S. et al., Molecular Basis and Clinical Application of Biological Markers of Bone Turnover, Endocrine Rev., 17:333-368, 1996).
In bone metastasis, markers of bone metabolism behave somewhat differently than in metabolic bone diseases such as osteoporosis. Among formative markers, increased PICP and BALP are observed in bone metastasis of prostatic cancer which is a typical example of bone hardening metastases but there is no significant increase in the level of osteocalcin which rises in osteoporosis and other metabolic bone diseases. The mechanism behind these phenomena is not presently known. In breast cancer which involves bone metastasis of a mixed type, the levels of formative markers increase but not as much as in prostatic cancer. In lung cancer which involves many cases of bone metastasis of a dissolution type, there are no significant increases in the levels of formative markers.
Among resorptive markers, ICTP differs from the other bone metabolic markers in that it does not change greatly after menopause but it has been found to increase in bone metastasis of cancer. From the viewpoint of detecting bone metastasis, ICTP may be considered a good marker that is insensitive to enhanced bone resorption in the post-menopausal stage. The levels of resorptive markers increase not only in bone metastasis of lung cancer which is mostly of a dissolution type but also in bone metastasis of breast cancer which is mostly of a mixed type, as well as in bone metastasis of prostatic cancer which is of a hardening type.